B.C. Supreme Court Justice James Williams faced the decision of a lifetime when the jury at Robert Pickton's murder trial returned with a question on its sixth day of deliberations.

The jurors asked if they could they still convict the Vancouver pig farmer if they decided that he had not acted alone in the gruesome murders of six Vancouver prostitutes.

After three hours of reflection and heated debate with Crown and defence lawyers, Judge Williams said that they could indeed find Mr. Pickton guilty, if he killed the women "or was otherwise an active participant" in the killings.

Soon afterward, the jury convicted Mr. Pickton on all six counts.

On Thursday, that pivotal episode comes under the piercing scrutiny of nine judges of the Supreme Court of Canada. Hanging in the offing is the fate of a man believed to be Canada's most prolific serial killer and the prospect of restaging an arduous case that consumed five years, featured 129 witnesses and included 1.3 million pages of documents.

As they climb the steps to the Supreme Court building, B.C. Crown counsel Gregory Fitch and John Gordon will have good reason to be nervous. Jury instructions are a leading cause when verdicts are overturned - and responses to jury questions are particularly vulnerable under review.

The fragility of the Pickton convictions was underlined last year, when the B.C. Court of Appeal split 2-1 in upholding his conviction.

In a strongly-worded brief to the Supreme Court, the B.C. prosecutors state that there was overwhelming evidence against Mr. Pickton.

"Ordering a new trial in this case, where conviction is inevitable, would only serve to detract from society's perception of fairness and the proper administration of justice," they said.

Retrying Mr. Pickton would be costly and futile, the brief stated.

In response, defence counsel Gil McKinnon argued that Judge Williams should not have permitted the Crown to recast its theory of the crime, from that of Mr. Pickton acting alone to the possibility of accomplices, to take advantage of the jury's quandary.

"Trial fairness is at the heart of this appeal," Mr. McKinnon said. "No matter how heinous the crime, an accused has a constitutional right to a fair trial."

In the five years before the case came to trial, it was marked by a painstaking police search of Mr. Pickton's farm that uncovered human remains, DNA and personal objects allegedly connecting Mr. Pickton to more than two dozen murders.

As the trial began, the Crown strategy focused on Mr. Pickton being the sole killer. It alleged that he lured the women to his farm, shot them execution-style, butchered their bodies in his slaughterhouse and buried the remains in different locations.

In an unusual development, the Crown is expected to disown that position tomorrow, instead argue that it is irrelevant whether Mr. Pickton was the sole killer or the self-described "head honcho" of a murderous venture.

Mr. McKinnon intends to take dead aim at this prosecutorial change of heart. His brief invokes a clamour that ensued when the Pickton jury emerged from its deliberations to ask the accomplice question.

In the absence of the jurors, Judge Williams slammed his hand down on his desk in frustration and said that his original instruction about the Crown's obligation to show that Mr. Pickton was the killer "had trouble written on it right at the time. I went with it because both counsel said that was the instruction that should be given."

At the time, Judge Williams pointedly reminded the Crown that it had agreed to the original instruction based on its theory.

Mr. McKinnon's brief accuses Judge Williams of acting precipitously and confusing the jury in his response. "On three occasions, the trial judge acknowledged that he did not understand the jury's question," it states. "However, he did not seek clarification from the jury."

Mr. McKinnon also noted that the Crown had flatly rejected suggestions from the defence during the trial that there were alternate suspects.

"The Crown ridiculed the defence position that other persons were responsible for the murders as 'straw men,' 'red herrings' and 'bogeymen,' " Mr. McKinnon said.

"Put simply, the jury's critical question was incorrectly answered, the 'goal posts' were changed by the amendment at a very late and impermissible stage of the trial. The Crown gained a significant unjustified advantage."

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Acted alone or 'actively participated?

The Crown theory at Robert Pickton's murder trial was that he shot or strangled six sex-trade workers on his pig farm in Coquitlam, B.C.

In December, 2007, a jury agreed, convicting Mr. Pickton of the second-degree murder of Sereena Abotsway, Mona Wilson, Andrea Joesbury, Marnie Frey, Brenda Wolfe and Georgina Papin. He was sentenced to life in prison with no parole eligibility for 25 years.

The key issue at his appeal will be whether Mr. Pickton's trial judge misdirected the jury after it emerged from the jury room to ask whether it could convict Mr. Pickton even if it believed that he was helped by an accomplice.

The trial judge told the jurors that they could; that it would be sufficient for them to conclude that Mr. Pickton had "actively participated" in the murders.

However, this instruction contradicted the trial judge's original admonition - delivered prior to the jury deliberations commencing - which stated that the Crown had to prove that Mr. Pickton had actually killed the victims.

Mr. Pickton still faces another 20 counts of first-degree murder. The B.C. Crown has indicated that it will prosecute if a new trial is ordered on the original six murder charges.